1. Field of the Invention
The present invention relates to a brake system having a master cylinder for generating brake fluid pressure based on a brake operation of a driver, and an electrical fluid pressure generation device for generating brake fluid pressure by an electrically controllable actuator.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2003-137084 discloses a brake system of the type referred to as a brake by wire (BBW) brake system, which converts a brake operation of a driver into an electrical signal used to operate electrical fluid pressure generation device, and operates a wheel cylinder by brake fluid pressure generated by the electrical fluid pressure generation device.
As seen in FIG. 7, which is a reproduction of FIG. 1 of Japanese Patent Application Laid-open No. 2003-137084, the known BBW brake system is constituted by connecting in series a fail-safe cylinder 2 operated through a brake pedal 1 to generate a brake fluid pressure, a master cylinder 4 operated by a motor 5 to generate brake fluid pressure, an anti-lock brake system (ABS) actuator 7 having a function of reducing, maintaining and increasing brake fluid pressure with respect to each vehicle wheel, and four wheel cylinders 8a to 8d. In a normal situation where the motor 5 can operate, the wheel cylinders 8a to 8d are operated by the brake fluid pressure generated by the master cylinder 4 to generate brake fluid pressure. In an abnormal situation where the motor 5 cannot operate, the master cylinder 4 is operated by the brake fluid pressure generated by the fail-safe cylinder 2 to generate brake fluid pressure.
The master cylinder 4 comprises first and second pistons 4a and 4b, first and second chambers 4c and 4d defined in front of the first and second pistons 4a and 4b, a piston rod 4e connected to the first piston 4a on the rear side and extending rearward, a fail-safe piston 4g slidably fitted to the rear part of a flange portion 4h provided on the piston rod 4e, and an on-fail pressurization chamber 4j which is defined in the rear of the fail-safe piston 4g and pressurized at the event of failure. A second chamber 2g of the fail-safe cylinder 2 communicates with the on-fail pressurization chamber 4j through a fail-safe valve 9.
When the fail-safe valve 9 is opened in a situation where the motor 5 for the master cylinder 4 becomes unable to operate due to power failure or the like, fluid pressure generated in the second chamber 2g of the fail-safe cylinder 2 is supplied to the on-fail pressurization chamber 4j via the fail-safe valve 9 to advance the fail-safe piston 4g, and thus the first and second pistons 4a and 4b advance together with the piston rod 4e having the flange portion 4h engaged with the fail-safe piston 4g, thereby generating brake fluid pressure in the first and second chambers 4c and 4d to exert a fail-safe function.
However, in the system described in Japanese Patent Application Laid-open No. 2003-137084, the fail-safe cylinder 2 and the master cylinder 4 communicate with each other substantially through only one fluid passage (the fluid passage in which the fail-safe valve 9 is provided). Therefore, there is a fear of complete failure of the fail-safe function if drainage or clogging occurs in the fluid passage, disadvantageously leading to a need for countermeasures such as preparation of a backup power source.